U.S. patent number 9,963,371 [Application Number 14/711,288] was granted by the patent office on 2018-05-08 for thermo-oxidation of municipal wastewater treatment plant sludge for production of class a biosolids.
This patent grant is currently assigned to U.S. Environmental Protection Agency. The grantee listed for this patent is Pegasus Technical Services, Inc., The United States of America as Represented by the Administrator of the U.S. Environmental Protection Agency. Invention is credited to Richard C. Brenner, Robert J. Grosser, Edith L. Holder, Makram T. Suidan.
United States Patent |
9,963,371 |
Suidan , et al. |
May 8, 2018 |
Thermo-oxidation of municipal wastewater treatment plant sludge for
production of Class A biosolids
Abstract
A process for treatment of municipal wastewater plant sludge to
the criteria of Class A biosolids. The process uses hydrogen
peroxide and thermo-oxidation to reduce volatile suspended solids
to meet the criteria. On a batch basis, waste activated sludge is
introduced into a reactor; the concentration of the waste activated
sludge is adjusted to about 1.5% total suspended solids with
secondary effluent, if necessary; the reactor is mixed; the reactor
is pre-heated to an operating temperature in a range of about
65.degree. C. to about 90.degree. C.; subsequently, a 50% solution
of laboratory grade hydrogen peroxide is introduced into the bottom
of the reactor; and the contents are heated for at least 4
hours.
Inventors: |
Suidan; Makram T. (Cincinnati,
OH), Brenner; Richard C. (Cincinnati, OH), Holder; Edith
L. (Cincinnati, OH), Grosser; Robert J. (Cincinnati,
OH) |
Applicant: |
Name |
City |
State |
Country |
Type |
Pegasus Technical Services, Inc.
The United States of America as Represented by the Administrator of
the U.S. Environmental Protection Agency |
Cincinnati
Washington |
OH
DC |
US
US |
|
|
Assignee: |
U.S. Environmental Protection
Agency (Washington, DC)
|
Family
ID: |
54537944 |
Appl.
No.: |
14/711,288 |
Filed: |
May 13, 2015 |
Prior Publication Data
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Document
Identifier |
Publication Date |
|
US 20150329393 A1 |
Nov 19, 2015 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61996629 |
May 13, 2014 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C02F
3/12 (20130101); C02F 11/18 (20130101); C02F
11/06 (20130101); C02F 2203/00 (20130101); C02F
2209/02 (20130101); C02F 2209/06 (20130101); C02F
1/722 (20130101) |
Current International
Class: |
C02F
1/72 (20060101); C02F 11/06 (20060101); C02F
11/18 (20060101); C02F 3/12 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Dhar et al., "Thermo-oxidative pretreatment of municipal waste
activated sludge for volatile sulfur compounds removal and enhanced
anaerobic digestion," Chemical Engineering Journal, 174 (2011)
166-174. cited by examiner.
|
Primary Examiner: Stelling; Lucas
Attorney, Agent or Firm: Stein IP, LLC Sullivan; Mark
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a non-provisional of U.S. Provisional
Application No. 61/996,629, filed May 13, 2014 in the U.S. Patent
and Trademark Office. All disclosures of the document named above
are incorporated herein by reference.
Claims
What is claimed is:
1. A process for treatment of municipal wastewater plant sludge
comprising: introducing waste activated sludge and/or thickened
mixed liquor sludge into a reactor; adjusting the concentration of
the waste activated sludge and/or thickened mixed liquor sludge to
about 1.5% total suspended solids with secondary effluent; mixing
the contents in the reactor; heating the reactor to an operating
temperature in a range of about 75.degree. C. to about 90.degree.
C.; subsequently introducing a 50% solution of hydrogen peroxide
into the bottom of the reactor; and heating the contents for at
least 4 hours to maintain the operating temperature within the
range.
2. The process of claim 1, wherein the hydrogen peroxide is
introduced to a concentration range of about 0.05 to 0.2 g/g
volatile suspended solids.
3. The process of claim 1, wherein the hydrogen peroxide is
introduced over about the first 30 minutes after reaching reactor
operating temperature.
4. The process of claim 1, wherein the hydrogen peroxide is
technical grade.
5. The process of claim 1, wherein the reactor is heated to an
operating temperature of about 90.degree. C.
6. The process of claim 2 wherein the hydrogen peroxide is
introduced to a concentration range of about 0.1 to 0.2 g/g
volatile suspended solids.
7. The process of claim 1, further comprising the step of producing
biosolids from the waste activated sludge and/or thickened mixed
liquor sludge.
8. The process of claim 7, wherein the biosolids produced by the
process meet Class A sludge biosolids regulations.
9. The process of claim 7, wherein fecal coliform levels in the
biosolids are non-detectable and do not regrow in 7 days.
10. The process of claim 7, wherein the biosolids settle to a
blanket level of about 150 mL to about 400 mL after 24 hours in a
1,000 mL graduated cylinder.
11. The process of claim 7, wherein a portion of the ammonia
nitrogen inventory in the waste activated sludge or thickened mixed
liquor sludge is released from the biosolids during treatment to
the liquid phase for recycle to the headworks of a wastewater
treatment plant.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
Aspects of the present invention relate to a heated chemical
treatment of wastewater treatment plant (WWTP) excess aerobic
sludge, including aerobically digested sludge, waste sludge from
oxidation ditches and other long-sludge residence time (SRT)
activated sludge wastewater treatment processes, and waste sludge
from conventional SRT activated sludge processes.
2. Description of the Related Art
Municipal WWTP sludge is typically composed of a combination of raw
primary sludge and excess or waste activated sludge that is
digested, either anaerobically or aerobically, to achieve solids
mass reduction, vector attraction reduction, and a reduction in
microbial indicators of fecal contamination such as fecal
coliforms. In most cases, the digested sludge is subjected to
mechanical dewatering to produce a drier material that can be
incinerated, disposed of in a sanitary landfill, or applied in bulk
to agricultural land as biosolids. Some producers of biosolids
further dry the processed material to the point where it can be
bagged and sold as a commercial soil conditioner/fertilizer (e.g.,
Milorganite produced by the Milwaukee Metropolitan Sewerage
District).
WWTP sludge is generally processed to levels where it can meet
Federal Class B sludge regulations. The Class B regulations
represent the minimum levels of pathogen reduction that are
acceptable for land application of biosolids (i.e., treated WWTP
sludge). These regulations specify that wastewater sludge must be
treated by a process to significantly reduce pathogens (PSRP) that
will achieve a vector attraction reduction (VAR) goal of 38%
reduction in volatile suspended solids (VSS) or meet a fecal
coliform level in the processed sludge .ltoreq.2,000,000 MPN (Most
Probable Number)/g, or alternately .ltoreq.2,000,000 CFU (Colony
Forming Units)/g, based on the geometric mean of seven samples.
Some states require municipal WWTPs to meet both stipulations to
achieve a Class B rating. PSRPs include, among others, anaerobic
sludge digestion at a mean cell residence time (MCRT) of at least
15 days at a temperature of 35.degree. C.-55.degree. C. and aerobic
sludge digestion at a MCRT of at least 40 days at 20.degree. C.
Land application of Class B biosolids, although widely practiced in
the United States, has been accompanied by numerous and ongoing
public complaints over the years. These complaints range from
emanation of malodors from the applied fields to claims of
illnesses and even deaths caused by volatilization of harmful
compounds contained in the biosolids or direct contact with the
biosolids. These complaints can be circumvented and most likely
dispelled by the land application of biosolids treated to a higher
level, namely Class A biosolids. The definition of Class A
biosolids mandates the reduction of fecal coliforms and/or
Salmonella to non-detect levels.
Prior research was conducted on anaerobically digested sludge
produced on site in short-term 5-day MCRT bench-scale digesters at
the University of Cincinnati (UC) (Cacho Rivero, 2005). Feed to the
anaerobic digesters consisted of a mixture of primary and waste
activated sludges from municipal WWTPs. The effluent sludge from
these digesters was treated in a thermo-oxidation process in
separate heated reactors. Hydrogen peroxide (H.sub.2O.sub.2) was
added at doses ranging from 0.1-0.5 g/g volatile suspended solids
(VSS) (dry wt.) and temperatures ranging from 35.degree.
C.-90.degree. C. The higher doses and temperatures produced the
greatest reduction in VSS. For example, at 90.degree. C., VSS
reductions of 58%, 65%, and 73% were achieved at H.sub.2O.sub.2
doses of 0.1, 0.25, and 0.5 g/g VSS, respectively. All of these VSS
reduction levels are substantially greater than the minimum 38%
reduction required for Class B sludge. The H.sub.2O.sub.2 dose was
bled into the reactor over 6 hours to minimize foaming. The pH of
the thermo-oxidation sludge remained largely unchanged, tending to
increase slightly. At 90.degree. C., no fecal coliforms were
detected in the H.sub.2O.sub.2-treated sludge, thereby meeting the
criteria for Class A biosolids.
Historically, WWTP design has utilized a two-stage treatment system
configuration with a first-stage primary settling process followed
by a second-stage biological treatment process. In the past, most
WWTPs have utilized conventional activated sludge designs with SRTs
in the range of 3-8 days as the second stage. Recently,
particularly for WWTPs with low to moderate hydraulic capacity
(i.e., 1-20 million gallons per day [mgd]), design engineers have
determined it is more cost effective to eliminate first-stage
primary settling of influent wastewater. Rather, influent
wastewater is fed directly to a longer-SRT (>15 days) extended
aeration activated sludge reactor, thereby obviating the need and
cost of handling combined primary and waste activated sludges.
Eliminating primary clarification in the treatment train and
further because activated sludge reactors produce only aerobic
sludge, there is less incentive to incorporate anaerobic digestion
in the sludge treatment flowsheet.
Based on the above evolution in WWTP design philosophy, emphasis
has shifted to the development of cost-effective methods for
treating excess sludge from aerobic systems. It was postulated that
the above thermo-oxidation concept would also perform well on
excess activated sludge to produce Class A biosolids.
The theory behind the mating of first-stage biological treatment
with follow-on second stage thermo-oxidation (chemical) treatment
is to use the microorganisms in the biological treatment stage to
cost-effectively oxidize most of the easy-to-degrade organics
contained in the sludge matrix and to use the more expensive
chemical (H.sub.2O.sub.2) treatment to oxidize the more
recalcitrant organic compounds that are not easily degraded
biologically. This treatment sequence optimizes what the biological
and chemical stages do best and most efficiently. Highly oxidized
excess sludge from WWTPs, whether produced in an aerobic digester
or as mixed liquor sludge in an extended aeration activated sludge
plant, and possibly even mixed liquor in a less oxidized
conventional activated sludge process, are suitable for direct feed
into the thermo-oxidation reactor. The thermo-oxidation process
should be able to accommodate most sludges typically produced by
municipal WWTPs.
Another benefit of the thermo-oxidation process is that some
fraction of the nitrogen (particularly ammonia) inventory in the
H.sub.2O.sub.2 feed sludge is solubilized during treatment in the
thermo-oxidation reactor and can be recycled to the head of the
treatment plant works in the reactor supernatant. If this did not
happen, the entire nitrogen load would be transported to the
application field in the biosolids. A significant fraction of this
load, particularly the easily released ammonia component, would be
rapidly solubilized and discharged into the soil, potentially
exceeding the sorption capacity of the soil and contaminating
ground water resources. By removing the easily released nutrient
components in the WWTP sludge, the nutrients more tightly bound to
the biosolids will be released slowly as needed for soil
conditioning and fertilization.
SUMMARY OF THE INVENTION
Aspects of the claimed invention overcome deficiencies in the prior
art.
Other aspects of the claimed invention provide a thermo-oxidation
process to cost effectively produce Class A biosolids from WWTP
excess sludges.
Further aspects of the claimed invention provide a process that can
effectively treat either excess sludge from an aerobic digester or
thickened mixed liquor from an extended aeration (long-SRT)
reactor.
The thermo-oxidation process described herein uses H.sub.2O.sub.2
addition at elevated temperatures to achieve increased levels of
VSS destruction and VAR and disinfection of excess sludge that has
been generated in an aerobic wastewater treatment process.
The thermo-oxidation process is operated by batch feeding waste
activated sludge into a constantly stirred tank reactor (CSTR). The
reactor is pre-heated to the target temperature between 65.degree.
C. and 90.degree. C. Technical grade H.sub.2O.sub.2 is slowly
introduced at or near the bottom of the reactor at a concentration
between 0.1 to 0.2 g/g VSS over the first 30 minutes of operation
to prevent foaming. When operating at 90.degree. C., a reactor
residence time of 2 to 4 hours is necessary to achieve maximum VSS
destruction. A reactor residence time 1 hour achieved non-detect
levels of fecal coliforms without regrowth potential. Substantially
improved settling characteristics are achieved with H.sub.2O.sub.2
treated sludge as measured by 30 minutes of settling in a 1,000 mL
graduated cylinder compared with sludge either untreated or treated
with temperature alone. Nitrogen in the form of ammonia is released
to the liquid phase where it can be recycled back to the head of
the WWTP to avoid rapid release in the soil and potential
contamination of ground water.
Additional aspects and/or advantages of the invention will be set
forth in part in the description which follows and, in part, will
be obvious from the description, or may be learned by practice of
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
These and/or other aspects and advantages of the invention will
become apparent and more readily appreciated from the following
description of the embodiments, taken in conjunction with the
accompanying drawings of which:
FIG. 1 illustrates the configuration for one bench-scale test
reactor according to an aspect of the claimed invention;
FIG. 2A shows the performance (% VSS removal) of the bench-scale
reactor on Mason, OH WWTP waste activated sludge (WAS) at various
temperatures and no H.sub.2O.sub.2 according to another aspect of
the claimed invention;
FIG. 2B shows the performance (% VSS removal) of the bench-scale
reactor on Mason, OH WWTP waste activated sludge (WAS) at various
temperatures and 0.05 g H.sub.2O.sub.2/g of VSS according to
another aspect of the claimed invention;
FIG. 2C shows the performance (% VSS removal) of the bench-scale
reactor on Mason, OH WWTP waste activated sludge (WAS) at various
temperatures and 0.1 g H.sub.2O.sub.2/g of VSS according to another
aspect of the claimed invention;
FIG. 2D shows the performance (% VSS removal) of the bench-scale
reactor on Mason, OH WWTP waste activated sludge (WAS) at various
temperatures and 0.2 g H.sub.2O.sub.2/g of VSS according to another
aspect of the claimed invention; and
FIG. 3 shows summertime performance (% VSS removal) for four other
municipal WWTPs in the Greater Cincinnati (OH) area.
DETAILED DESCRIPTION OF THE EMBODIMENTS
Reference will now be made in detail to the present embodiments of
the present invention, examples of which are illustrated in the
accompanying drawings, wherein like reference numerals refer to the
like elements throughout. The embodiments are described below in
order to explain the present invention by referring to the
figures.
To achieve reduction of VSS on a batch basis, waste activated
sludge (WAS) is introduced into a reactor; the concentration of the
WAS is adjusted to about 1.5% total suspended solids (TSS) with
secondary effluent, if necessary; the reactor is mixed; the reactor
is pre-heated to an operating temperature in a range of about
65.degree. C. to about 90.degree. C.; subsequently, a 50% solution
of laboratory grade H.sub.2O.sub.2 is introduced into the bottom of
the reactor; and the contents are heated for at least 4 hours.
The H.sub.2O.sub.2 is introduced to the reactor upon reaching the
operating temperature at a concentration range of about 0.05 to 0.2
g/g VSS over about 30 minutes to reduce foaming. Evaporation is
controlled by directing pressurized air through an air humidifier
into the head space of the reactor at a flow rate of about 200
mL/minute and/or by installing a condenser on the reactor to
condense and recycle water vapor in the head space.
Experiments were conducted in a laboratory (see below), and four
waste treatment plant test reactors were operated in parallel
during data collection runs. Different operating conditions were
imposed on each reactor during each run. The following operational
description applies to one of the test reactors. The configuration
described is illustrated in FIG. 1.
The test reactor 1 was a 4-L glass flask. A rubber stopper (not
numbered) was inserted into the neck of the flask resulting in a
sealed reactor. Two L of WAS at a concentration of approximately
1.5% TSS were batch fed into the test reactor 1. To obtain a
concentration of roughly 1.5% TSS, the thickened WAS feed was
diluted with secondary effluent from the same WWTP. The test
reactor was set on a stirring plate 2, and a 4-in. stir bar 3 was
placed into the flask. The stirring plate 2 was turned on to bring
the stir bar into motion. The stirring plate 2 was maintained at a
setting that would keep the stir bar 3 in uniform circular motion
on the bottom of the reactor 1 to promote mixing of the WAS
contents.
A temperature controller 5 (in this embodiment a rheostat wired to
a thermocouple) was used to control liquid temperature within the
test reactor. The thermocouple was inserted through the rubber
stopper into the test reactor 1 contents. The test reactor 1 was
wrapped with heat tape overlain with glass wool insulation 4
(hereinafter heat tape and/or glass wool insulation 4). The heat
tape 4 was also wired to the temperature controller 5. The
temperature controller 5 rheostat was set at the desired test
reactor 1 operating temperature. The desired test reactor 1
operating temperature was maintained via a signal from the
thermocouple to the temperature controller 5 rheostat to control
the current to the heat tape 4. To verify the accuracy of the
system, temperature readings inside the test reactor 1 were also
checked periodically with a thermometer 6, also inserted through
the rubber stopper.
Pressurized laboratory air 13 was directed through an air
humidifier 9 and then through an opening in the rubber stopper and
into the head space of the test reactor 1 (the head space is dotted
line above the test reactor 1. The purpose of the humidified air
injection was to prevent water loss from the test reactor 1 during
its operation. The flow rate of the pressurized laboratory air 13
was maintained at .about.200 mL/minute via an inlet valve installed
in the feed line.
Another control to prevent loss of water from the reactor was the
use of a condenser 10 that received cold water from a refrigerated
bath 12. Water vapor in the headspace condensed and flowed back
into the test reactor 1. In this way, evaporation was minimized
during a test run. Exit gas flowed from the condenser into an
Erlenmeyer flask 11 containing water to monitor gas flow and
prevent backflow.
For those reactors receiving H.sub.2O.sub.2 treatment, a 50%
solution of laboratory grade H.sub.2O.sub.2 was injected into the
bottom of the test reactor 1 using a syringe pump 8. The syringe
pump 8 was fitted with a 10-mL syringe 7 containing the
H.sub.2O.sub.2 dose. The selected H.sub.2O.sub.2 feed dose was
pumped into the test reactor 1 over a 30-minute period (to minimize
foaming) beginning immediately after the test reactor 1 WAS
inventory reached its targeted operating temperature for that run.
Calculation of the H.sub.2O.sub.2 dose was made from VSS
measurement of the feed WAS.
The temperature controller 5 was activated immediately following
loading of the test reactor 1 with WAS. Depending on the target
operating temperature (65.degree. C.-90.degree. C.), the test
reactor 1 typically reached that temperature within 30-60 minutes.
At this time, injection of the selected H.sub.2O.sub.2 dose was
initiated using the syringe pump 8. WAS samples were usually
collected at t=initial (when the temperature controller 5 was
turned on), t=0 (when the test reactor 1 temperature reached its
target level), t=1 hour, t=2 hours, t=4 hours, t=8 hours, and t=24
hours. Analyses and measurements conducted on these samples
consisted of TSS, VSS, chemical oxygen demand (COD), total Kjeldahl
nitrogen (TKN), ammonia nitrogen (NH.sub.4--N), total phosphorus
(TP), and fecal coliforms. pH and test reactor 1 temperature were
monitored routinely throughout each run. Solids settling rates were
measured after 24 hours of operation when the test reactor 1 was
emptied by recording compacted sludge volume (or sludge blanket
level) in a 1,000-mL graduated cylinder after 30 minutes and 24
hours of settling.
Key parameters in defining process performance with this technology
are fecal coliform destruction and VSS reduction. Fecal coliforms
are a universally recognized indicator microorganism for the
presence or absence of pathogenic microorganisms. It is the key
microbiological parameter for determining if a treated biosolids
product meets Class A standards. If fecal coliforms are absent, the
assumption is that pathogens also are not present. VSS reduction is
an indirect measure of the amount of particulate organic matter
oxidized during sludge treatment. VSS reduction is the sole
mechanism with this process by which sludge mass is decreased for
minimizing sludge handling cost. It is also critical in achieving
VAR and a stable sludge mass that can be applied to land or stored
awaiting land application without the threat of objectionable odor
generation.
FIGS. 2A-2D are plots of VSS removal vs. reactor residence time for
triplicate runs conducted on Mason, OH WWTP WAS. Four conditions
are plotted showing no H.sub.2O.sub.2 addition (a temperature only
control) and H.sub.2O.sub.2 doses of 0.05, 0.1, and 0.2 g/g VSS.
Three operating temperatures (65.degree. C., 75.degree. C., and
90.degree. C.) were evaluated for each H.sub.2O.sub.2 dose. All
operating temperatures for a given H.sub.2O.sub.2 dose are shown on
each graph. Values for t=initial, t=0, and t =1, 2, 4, 8, and 24
hours are given. As would be expected, three trends are indicated.
VSS removal (i.e., organic matter destruction) increases with
increasing reactor residence time, increasing operating
temperature, and increasing H.sub.2O.sub.2 dose. For the 75.degree.
C. and 90.degree. C. temperatures, 86%-92% of the total 24-hour VSS
reduction was achieved in the first 4 hours for the 0.1 and 0.2
H.sub.2O.sub.2 doses. At these two temperatures for the 0.05
H.sub.2O.sub.2 dose, VSS removal dropped to 80%-84% of the ultimate
24-hour removal in the first 4 hours. In other words, the major
fraction of VSS removal was obtained for these two operating
temperatures at the two highest H.sub.2O.sub.2 doses in the first 4
hours of operation; somewhat less removal was achieved at the
lowest H.sub.2O.sub.2 dose. For the 65.degree. C. operating
temperature, VSS removals achieved in the first 4 hours dropped to
75%-85% of their respective 24-hour removals at the various
H.sub.2O.sub.2 doses.
For the 90.degree. C. operating temperature, VSS removal roughly
doubled with the 0.2 H.sub.2O.sub.2 dose for the 4-hour (26% to
55%) and 24-hour (33% to 62%) residence times compared to the
undosed controls during cold weather. During warmer weather, VSS
destruction increased 65% (19% to 31%) at this dose for the 4-hour
residence time (no data were generated for a 24-hour residence
time). Less VSS reduction, as expected, is achieved during warmer
weather as wastewater temperature increases. More of the influent
wastewater organics in warmer weather are oxidized in the secondary
treatment activated sludge aeration tank before reaching the excess
sludge handling process. At 90.degree. C., the incremental
differences in VSS removal at the two residence times with the
addition of H.sub.2O.sub.2 at 0.1 g/g VSS vs. no addition tended to
approximate half of the incremental differences noted at 0.2 g/g
VSS. At an H.sub.2O.sub.2 dose of 0.05 g/g VSS and 90.degree. C.,
the incremental differences dropped to about one-fourth of those
achieved with the highest dose.
In FIG. 3, % VSS vs. reactor residence times plots are shown for
four WWTPs, other than the Mason plant, in the Greater Cincinnati
area. All test runs were conducted during warm weather on WAS at an
H.sub.2O.sub.2 dose of 0.2 g/g VSS (along with undosed controls)
and an operating temperature of 90.degree. C. The results for three
(Sycamore, Harrison, and Mill Creek) of the four WWTPs were quite
close. The data curve for the fourth WWTP (Little Miami) while
somewhat lower than the curves for the other three plants had the
same approximate shape. These data confirmed the findings from the
Mason test runs, namely that the large majority of VSS removal is
achieved in the first 4 hours of operation and incremental VSS
reduction with the 0.2 H.sub.2O.sub.2 dose at 90.degree. C. can
approximate up to twice the incremental VSS reduction noted with no
H.sub.2O addition.
Fecal coliform values decreased to non-detectable levels after 1
hour (or less) of reactor residence time at all three temperatures
tested and with all three applied H.sub.2O.sub.2 doses. No regrowth
was noted after 1 week at room temperature with any of these
samples. Reduction of fecal coliforms to non-detectable levels
occurred with heat alone for the 75.degree. C. and 90.degree. C.
conditions, but not at 65.degree. C. In some instances, subsequent
regrowth at room temperature was observed.
This thermo-oxidation process also greatly enhances sludge
settleability. In numerous post treatment sludge settling tests,
WAS that was treated with heat only would settle to a blanket level
of .about.700-980 mL in a 1,000-mL graduated cylinder after 24
hours. In contrast, WAS that was both heated and dosed with
H.sub.2O.sub.2 would settle to a blanket level of .about.150-400 mL
in a 1,000-mL graduated cylinder after 24 hours.
The above data form the basis for the recommended operating
conditions for this invention. VSS reductions achieved at an
operating temperature of 65.degree. C. are too low to be cost
effective. Operation at this temperature is not recommended.
Significant VSS reductions were observed at both 75.degree. C. and
90.degree. C., although, based on the data, the incremental
reduction over that of the control reactor achieved at 90.degree.
C. can be as much as twice that accomplished at 75.degree. C. For
optimum performance, operation at 90.degree. C. is recommended. As
acceptably high levels (.gtoreq.90%) of VSS reduction are achieved
within the first 4 hours of operation vs. that achieved at 24
hours, operation at a reactor residence time of 4 hours is also
recommended. This reactor residence time is eight times longer than
the minimum 30 minutes sludge must be held at 90.degree. C. to meet
Class A biosolids regulations. Finally, the invention user has a
choice of two acceptable H.sub.2O.sub.2 doses, 0.1 and 0.2 g/g VSS.
With the higher dose, incremental VSS reduction vs. that of an
undosed control will be up to two times higher than that of the
lower dose. The final selection should be based on a cost analysis
and the goals and requirements of the user.
All components of this bench-scale reactor can be readily adapted
to a full-scale system. Most equipment can be purchased
off-the-shelf. Only the heat exchange system may have to be custom
designed. Standard corrosion-resistant tankage can be used for the
system reactor.
Although a few embodiments of the present invention have been shown
and described, it would be appreciated by those skilled in the art
that changes may be made in this embodiment without departing from
the principles and spirit of the invention, the scope of which is
defined in the claims and their equivalents.
* * * * *